FIG. 12 is a top view of a conventional optical ranging sensor, and FIG. 13 is a cross sectional view of FIG. 12 taken along an A-A line.
As shown in FIG. 13, the optical ranging sensor includes one light emitting device 101, one PSD (Position Sensitive Detector) 102 which is a light receiving device for position detection, a light emitting-side lens 103, a light receiving-side lens 104, and one IC (Integrated Circuit) 105 which is configured to process a signal outputted from the PSD 102 and drive the light emitting device 101 at a predetermined timing. The PSD 102 has only one light receiving section, and a resistance of the light receiving section is uniform and constant within the light receiving section.
In the case where ranging is conducted based on triangulation by the conventionally-structured optical ranging sensor, because this optical ranging sensor has only one light receiving section in the PSD 102 (see FIG. 13) and the resistance of the light receiving section is uniform and constant within the light receiving section, distance measurement precision is deteriorated when a distance to an object is long as explained later.
FIG. 14 is a view for explaining the principle of the triangulation system.
It is to be noted that in FIG. 14, a relation of X=A·(f/d) holds, where X represents a size of an image, A represents a center-to-center distance between the light emitting-side lens 103 and the light receiving-side lens 104, f represents a focal length of the light receiving-side lens 104, and d represents a distance from a plane including the center of the light emitting-side lens 103 to an object 100.
In the optical ranging sensor which detects the distance d to the object 100 by the triangulation system, the distance d to the object 100 is in inverse proportion to an output V of the PSD 102, i.e., the size X of an image as shown in FIG. 16.
Therefore, when the distance d to an object is short, change in an output V due to distance fluctuation is large, whereas when the distance d to an object is long, change in an output V due to distance fluctuation becomes small, i.e., when the distance d to an object is short, distance measurement can be performed with sufficient precision, whereas when the distance d to an object is long, distance measurement is performed with poor precision.
In the triangulation system, the ranging precision depends on the center-to-center distance A between the light emitting-side lens 103 and the light receiving-side lens 104, and on the focal length f of the light receiving-side lens 104. More specifically, distance measurement precision is enhanced by increasing A and f. However, this entails a problem of increase of the size of the optical ranging sensor.
Moreover, when the distance d to an object is long, it is necessary to increase a lens diameter so as to prevent an insufficient light amount, and this may also cause the problem of increase of the size of the optical ranging sensor.
JP 2002-195807 A discloses an optical ranging sensor with use of a CMOS image sensor as a position detecting element. However, in this optical ranging sensor, the light emitting device and the light receiving device are not provided on the same plane and every element is independently packaged, which also causes the increase in size of the ranging sensor. In addition, a processing section for processing a signal taken out from the CMOS image sensor and a drive circuit section for driving the light emitting device are placed outside of a CMOS image sensor chip, and not all the elements and regions are mounted in one chip, which causes further increase in size of the ranging sensor. Furthermore, manufacturing of the optical ranging sensor needs a large number of process steps, and operation in each process step is complicated to respond to a need for precision, which in turn causes a problem of higher manufacturing costs.
As for the amount of light, this optical ranging sensor performs two exposures: the first exposure for detecting a light amount to adjust the light amount of the light emitting device and the next exposure for detecting a light amount for use in measurement. The amount of light become larger as a current passed through the light emitting device is increased. However, since supplying a current beyond a limit size to the light emitting device may lead to destruction of the light emitting device, considerable increase in supply current is not achievable. Because of this, considerable reduction in a lens diameter cannot be made, and therefore downsizing of the ranging sensor is difficult.